Materials having a high thermal conductivity are widely used in heat exchanging devices, such as heat sinks and heat exchangers etc, and are typically comprised of metals with high thermal conductivity, such as aluminum, copper and silver, the thermal conductivity of these metals being 120 to 220 W/mK, 400 W/mK and 430 W/mK, respectively. Silver is rather expensive and not used in the same extent as copper. The use of copper has some drawbacks. One is the rather high density (8.9 g/cm3) that makes devices using copper heavy. The high density of copper also results in a quite low value of the thermal diffusivity α=1.2 cm2/s (α=λ/Cp*ρ where α is the thermal diffusivity factor, λ is the thermal conductivity factor, Cp the thermal capacity and ρ the density). The low thermal diffusivity restricts the application of copper to articles where fast heat transfer is not needed. Another drawback is copper's tendency to oxidize. Copper oxide formed on the surface of a heat-exchanging device significantly reduces the thermal properties of the whole device. Yet another drawback is that copper has a high thermal expansion coefficient relative to material used in integrated circuits, the difference in thermal expansion coefficient causes tensions and a risk of cracks at the joint between the circuit and the heat sink if copper is used as heat sink material. The drawbacks of aluminum are a moderate thermal conductivity and a high thermal expansion coefficient.
In the electronic industry a need for better heat sinks have arisen due to the development of faster and smaller circuits. The heat producing elements can now be more closely packed. Consequently, the heat transfer has to be more efficient, the excess heat from local hotspots needs to be conducted away quickly The primarily requirements for heat sink material are high thermal conductivity, a thermal expansion coefficient close to that of Si and low specific gravity (see MRS Bulletin, volume 26 No 6,June 2001). Here diamond would have presented itself as the obvious material of choice for heat sinks.
Diamond is known to have good thermal conductivity properties (500 to 2000 W/mK) and would have been the perfect material for heat sinks would it not be for the cost and the difficulty of manufacturing suitable shapes. Still many heat sinks make use of diamond. There are different ways of applying diamond: as single diamond crystals, CVD diamond coatings and diamond composites.
U.S. Pat. No. 6,031,285 (Sumitomo) discloses a heat sink for semiconductors that has a structure which comprises a metal (A) of at least one metal selected from the group consisting of Cu, Ag, Au, Al, Mg, and Zn; a carbide (B′) made from a metal (B) of at least one metal selected from the group consisting of the groups 4a and 5a of the Periodic Table and chromium; and a plurality of diamond particles. The heat sink has a structure wherein more than a quarter of the surface of the individual diamond particles is covered with the metal carbide (B′) and the diamond particles covered with the metal carbide (B′) are separated from one another by the metal (A). The heat transport goes from the diamond to the metal (A). The thermal conductivity properties ranges from 230 to 730 W/mK, the lowest values is achieved when the metal A consist of Ag, Cu and Mg and the highest when the metal A consist of mostly Ag and a small amount of Cu. Shortcomings of the invention are: low thermal conductivity properties when using Al, Mg, and Zn, and cost when using Ag and Au.
U.S. Pat. No. 6,171,691 (Sumitomo) discloses a material where diamond particles are surrounded by metal carbide, the metal carbide and diamonds form a skeleton, the interstices in this skeleton are filled with metal. First a metal alloy containing a small amount of carbide former is infiltrated between the diamonds that are placed in a mould. The carbide-former reacts with the diamonds and forms carbide on the surface of the diamonds. The carbide together with the diamond forms a connected structure. The “carrier” metal alloy is either solidified between the carbide-covered diamonds or evaporated. Then a second metal alloy is infiltrated into the free space of the porous body. Then the body is taken out of the mould. The metal is at least one of Ag, Cu, Au and Al and the carbide-former is at least one of Ti Zr and Hf. The heat transport goes from the diamond through the carbide to the metal. The thermal conductivity property ranges from 300 to 900 W/mK Shortcomings of this material are complicated processing and high cost of the product.
Both U.S. Pat. No. 6,031,285 and U.S. Pat. No. 6,171,691 teach away from using the carbide former as the only filling element, the reason being the carbide forming element in itself has a low thermal conductivity, which would lower the thermal conductivity of the invented material as a whole.
U.S. Pat. No. 5,783,316 (University of California Oakland) discloses a diamond-copper-silver composite. The material consists of diamond particles bound together with copper or copper alloys. The thermal conductivity of this material lies between that of the diamond particles and that of copper. In order to get a better adhesion of the copper alloys to the diamond particles, the latter are covered with a thin layer of carbide-forming metals. Drawbacks of this material are the rather high density caused by copper and the high thermal linear expansion coefficient, which is also determined by copper. The high thermal expansion results in a substantial deformation of the article at elevated temperatures. Also the material, like pure copper itself, is not resistant aghast oxidation.
There exists a number of patents disclosing diamond composites where the intended use is not primarily heat exchange. The main fields of application for these types of material are cutting and abrasive wear components. Several patents reveal techniques to produce materials containing diamond, silicon carbide and silicon using high-pressure methods
U.S. Pat. No. 4,151,686 discloses a high pressure, high temperature method, in which high pressure is used during the sintering step in order to stay in the diamond stable area of the phase diagram at 1300-1600° C., the sintering being performed in high-pressure chambers with pressures of 30.000-60.000 atm. Specially made presses and dies only achieve the required extremely high pressures. The consequences are high production costs, limited production capacity and limited shapes and sizes of the diamond composite bodies. The material produced according to the teachings of U.S. Pat. No. 4,181,686 contains at least 80 vol. % up to 95 vol. % of diamonds with a large amount of diamond-to-diamond bonds. The high content of diamond makes the material hard but also brittle and sensitive for mechanical shocks.
Another material produced with high-pressure high temperature methods is Syndax3 from De Beers. It is a material intended for abrasive wear, such as rock drilling. The material consists of diamond particles and SiC sintered together. According to The Industrial Diamond Review No 61985 Syndax3 material has diamond-diamond contact. One might think that a diamond-diamond contact would be good for the thermal conductivity properties Even so, Syndax3 exhibits, according to our measurements, a thermal diffusivity factor of not more than 1.442 cm2/s and a thermal conductivity of not more than 265W/mK.
Several patents reveal techniques to produce materials containing diamond, silicon carbide and silicon without using high pressures. There are a number of variations of the process, mainly concerning the use of different carbonaceous materials (hereafter referring to all kinds of non-diamond carbon materials like carbon black, carbon fibers, coke, graphite, pyrolytic carbon etc). In principle the following steps are followed. A. Non-coated diamond particles or normally, carbon-coated diamond particles and carbonaceous materials are used as precursor materials. According to the examples, U.S. Pat. No. 4,220,455 starts with adding a thin layer (500-1000 Angstrom) of carbon on the diamonds by a pyrolytic reaction. The pyrolysis is done in vacuum for a few minutes by feeding natural gas or methane, into a furnace with diamond particles at 1200° C. Sometimes diamonds without a pyrolytic carbon layer are used, as in U.S. Pat. No. 4,381,271, EPO 0 043 541, EPO 0 056 596 and JP 6-199571A. Both carbon-coated and non-coated diamonds are mixed with carbonaceous materials as a main source of carbon, e.g. carbon black, short carbon fibers or cloth and a binder e.t.c, before the forming of green bodies. B. Forming of green bodies of the diamond particle/carbon material mixture is done in a mould. The green bodies contain additional solvents and temporary or permanent binders to facilitate the forming and to increase the strength of the green body. C. Work-pieces are made by heat-treating the green bodies. Some binders are vaporized without leaving any residues e.g. paraffin, other binders are hardened leaving a carbonaceous residue in the work-piece, e.g. phenol-formaldehyde and epoxy resins. D. Infiltration of the porous work-piece with molten silicon is done to form silicon carbide in a reaction between the carbon and the silicon. The heat treatment is done in such a manner as to minimize the graphitization of diamond, which is considered harmful. In the examples of U.S. Pat. No. 4,220,455 silicon is infiltrated in vacuum when the body is in a mould, at a temperature between 1400°-1550° C. for 15 minutes, during which time the reaction between silicon and carbon is completed. U.S. Pat. No. 4,242,106 uses a vacuum of 0,01-2,0 torr during the infiltration. The required time, depending largely on the size of the body is determined empirically and takes about 15-20 minutes at a temperature above 1400° C., or 10 minutes at 1500° C. U.S. Pat. No. 4,381,271 uses carbon fibers to promote the infiltration of liquid silicon by a capillary action. In most of the patents infiltration is made in a mould. In some earlier patents the infiltration is made outside the mould, like in EPO patent 0 043 541.
None of the methods to produce diamond-silicon carbide-silicon composites described above use graphitization intentionally.
In RU patent 2036779, a preform is moulded of diamond powder, possibly together with water or ethyl alcohol. The preform is placed in a furnace and impregnated with liquid silicon at 1420-1700° C. in argon or vacuum. In the process the surface of the diamond grains is minimally graphitized, such as the greater part of the diamond is still unchanged. This minor amount of graphite reacts in contact with infiltrated silicon, creating a thin surface of silicon carbide that keeps back any further formation of diamond to graphite during the used process. The drawback of this method is poor control and there is no method for governing the amount of produced SiC, residual silicon or residual porosity left in the composite.
In WO99/12866 and in WO000 18702, methods to produce a diamond-SiC—Si composite are disclosed. The composites produced consist of diamond particles in a matrix of SiC and Si or Si alloy in the following proportions: diamond particles at least 20 volume %, SiC at least 5 volume %. The composite has an excellent combination of properties such as low density, high elasticity modulus, low thermal expansion coefficient and it resists oxidation. However, the thermal conductivity of the material is not high enough to be able to solve the need of better heat sinks for the electronic industry.
The object of the present invention is to provide a material having at room temperature a thermal conductivity factor of at least 400 W/mK and a thermal diffusivity of at least 2.1 cm2/s, which could be produced in a desired shape and in a cost-efficient way.